Rotary hydraulic devices of the subject type generally include a housing, a rotor mounted for rotation within the housing, and a plurality of vanes individually slidably disposed in corresponding radially extending peripheral slots in the rotor. A cam ring radially surrounds the rotor, and has an inwardly directed surface forming a vane track and one or more fluid pressure cavities between the cam surface and the rotor. Inlet and outlet passages in the housing feed hydraulic fluid to and from the fluid pressure cavity or cavities.
U.S. Pat. No. 4,505,654 discloses a balanced dual-lobe rotary vane pump in which the rotor cavity is formed by the cam ring and side support plates, with relatively thin pressure plates, also referred to as cheek plates, valve plates or flex plates, disposed between the support plates and the rotor. A pocket in each support plate is surrounded by seals that engages the pressure plate to form a hydrostatic pressure pool or pad between each support plate and its adjacent pressure plate. The outlet passages from the pump chambers extend through the pressure pools, so that the pressure pools are filled with fluid at substantially outlet pressure. The fluid pressure in the hydrostatic pools urges the pressure plates inwardly toward the rotor to balance or slightly exceed the forces of fluid pressure in the pumping chambers, and the pressure distribution of leakage fluid that flows between the rotor and pressure plates. Terminal hole vane slots in the rotor cooperate with each vane to form under-vane chambers at the axial outer ends of each vane and an intra-vane chamber at an intermediate section of each vane. Passages and grooves in the pressure plates and radial holes in the rotor segments feed fluid at inlet pressure to the under-vane chambers, and fluid at outlet pressure to the intra-vane chambers, for urging the vanes radially outwardly against the cam ring. The radial holes in the rotor segments communicate the pressure at the inter-vane volume to the terminal hole vane slots to reduce the radial thrust force of the vanes on the cam surface.
Although rotary vane pumps and other hydraulic devices of the subject type have enjoyed substantial commercial acceptance and success, further improvements remain desirable. For example, although provision of the hydrostatic pressure pools as disclosed in the above-noted U.S. patent improves fluid pressure balance as compared with previous art, the pools are disposed adjacent to the outlet sections of the pumping chambers, and thus do not provide pressure support on the pressure plate areas adjacent to the pump inlet sections. This lack of axial support permits localized outward deflection of the pressure plate and increased leakage of the displaced volume. Another problem arises due to the varying number of vane/rotor segments of the rotating group disposed within each pressure chamber. In a ten-vane pump, for example, the number of vane/rotor segments in each pumping chamber alternates in a sequence two-three-two-three, etc. as the rotor rotates. The hydrostatic pressure pools are designed to provide an average hydrostatic pressure force equivalent to the separating pressure force of 2.5 vane/rotor segments at pressure per displacement cycle. The axial balance on the pressure plates is sensitive to operating conditions affecting inlet pressure and diminished performance is noticed. Another problem in the art lies in the audible noise and erosive wear associated with outgassing of the dissolved air when the pressure fluid is subjected to throttling during the precompression of the fluid volume entering the displacement chamber. Metering grooves at the pressure plate ports in the prior art provide single stage throttling which produces considerable outgassing. With multistage orificing, the precompression flow contains considerably less outgassing, which result in quieter operation and reduced erosive wear.
It is therefore a general object of the present invention to provide a rotary hydraulic device, particularly a vane pump, that exhibits improved operational integrity, improved efficiency, reduced audible sound level, improved consistency of performance, reduced sensitivity to speed variations and/or reduced sensitivity to operation at sub-atmospheric pressure. Another and more specific object of the present invention is to provide a rotary hydraulic device of the described character that exhibits improved balance of fluid pressure forces on the pressure plates at all phases of operation. A further object of the present invention is to provide a rotary hydraulic device, particularly a vane pump, that satisfies one or more of the foregoing objectives while being economical to assemble and reliable over an extended operating lifetime.